(1) Field of the Invention
The present invention relates to a light-emitting element driving device, and relates more particularly to a device that drives a light-emitting element such as a light-emitting diode (LED) connected to a power supply circuit.
(2) Description of Related Art
LEDs are increasingly used for backlights in liquid crystal display (LCD) panels. When LEDs are used as a backlight for an LCD panel (LCD backlight), a specific constant current is generally supplied to a plurality of LEDs connected in series, causing them to emit light. The number of LEDs and the amount of current supplied are determined according to the amount of required light. The drive voltage for driving the LEDs is produced by a voltage converter that converts the supply voltage to a specific voltage. This voltage converter controls the drive voltage by detecting the voltage or current at a specific part of the LED array (the load) in a feedback control loop. This type of LED drive technology is taught, for example, in Japanese Unexamined Patent Appl. Pub. JP-A-2008-130513.
The light-emitting element driving device taught in JP-A-2008-130513 is described briefly below with reference to FIG. 3.
The light-emitting element driving device according to this example of the related art detects the current supplied from a DC/DC converter 1 to the LED module 2 by means of a current detection resistor R1. A comparator 3 compares the detected voltage with a reference voltage Vref1, and based on the result of this comparison the PWM (pulse width modulation) controller 4 controls the DC/DC converter 1. A constant current supply can therefore be provided to the LED module 2. Control elements Q1 to Q3 rendering a current mirror circuit are also connected in series with the LED load circuits U1 to U3 in the LED module 2 to drive the LED load circuits U1 to U3 at a constant current level to achieve uniform light output. The voltage at the nodes between the control elements Q1 to Q3 and switches SW1 to SW3 (referred to as the “monitored voltage” below) is also monitored. Comparators CP1 to CP3 detect short circuit failure and open circuit failure of an LED by comparing the monitored voltage with a specific reference voltage Vref2. The failure controller 5 isolates the failed circuit by means of switches SW1 to SW3 and adjusts reference voltage Vref1 based on comparator output.
The light-emitting element driving device according to the related art described above detects LED failures by comparing the monitored voltage, which is the voltage at the node between each control element (also called a drive current generator) and switch with a fixed reference voltage. However, sudden load variations in the backlight system of a television using an LCD panel can produce overshoot and other voltage fluctuations in the drive voltage output by the DC/DC converter (also called a drive voltage generator). This fluctuation in the drive voltage may also cause the monitored voltage to vary. As a result, even though the LED is operating normally, operation of the comparator that compares the monitored voltage with the fixed reference voltage may cause the failure controller to operate incorrectly.